A light emitting diode (LED) is a semiconductor diode for radiating light, and has a function of converting an electrical energy into ultraviolet light or visible light. Particularly, for the purpose of utilizing the visible lights, there has been widely used a LED in which a light entitling chip formed of light emitting materials such as Gap, GaAsP, GaAlAs, GaN, InGaAlP or the like is sealed by a transparent resin. Further, there has been also widely used a display-type LED in which the light emitting material is fixed on an upper surface of a printed circuit board or a metal lead, and the fixed material is sealed by a resin case formed in a shape of numerical character or letter.
Further, when various phosphors are contained into a front surface of the light emitting chip or contained in the resin, it is also possible to suitably control the color of the emitted lights. That is, the LED can reproduce various lights in visible light region ranging from blue to red which is suitable for concrete use. Furthermore, since the light emitting diode (LED) is a semiconductor element, LED has a long life property and a high reliability, so that an exchanging frequency of LED due to failure is effectively decreased when the LED is used as a light source. Therefore, LED has been widely used as a constituting part of mobile communication devices, personal computer peripheral equipment, OA (office automation) devices, household electrical appliances, audiovisual apparatuses, various switches, various display devices such as light source display plate for backlight or the like.
However, in recent years, a sense of color of user utilizing the above various display devices has become highly upgraded, so that there has been technically demanded a function of capable of reproducing delicate color shade with a high resolution and a uniform external appearance of LED. In particular, LED for emitting white color light has grown in usage and popularity as a backlight for cellular phone or a lamp for automobile use, and demand of LED has been expected to greatly increased as an excellent substitute for fluorescent lamps. In this regard, in order to realize a high color-rendering property of the white light and the uniform external appearance of LED, various improvements have been tried.
Up to now, as LED for emitting white color light that have generally used or have been in trial run, there are two types of LEDs: LED which is formed by combining a blue luminescence diode (blue emission diode) with yellow luminescence phosphor (YAG), and If occasion demands, red phosphor (hereinafter referred to as “type 1”): and LED which is formed by combining a diode for emitting ultraviolet rays or ultraviolet lights with blue, yellow, red luminescence phosphors (hereinafter referred to as “type 2”). At the present stage, since the type 1 has a higher luminance than the type 2, the type 1 has been most widely used.
However, the type 1 has posed the .following drawbacks. Namely, the emitted light is liable to be recognized by user as yellowish light depending on a direction from which the user sees the emitted light, and unevenness of yellow color light and blue color light appear on a white surface when the emitted light is projected onto the white surface. Therefore, there may be a case where the light emitted from the white luminescence LED of type 1 is called as “pseudo-white light”.
When the white color light is evaluated by an average color rendering index which indicates a quality of the white color light, the average color rendering index of the white luminance LED of type 1 remains at low range of 70 to 75 (70 or more 75 or less).
On the other hand, although the white luminance LED of the latter (type 2) is inferior to the former (type 1) in luminance, unevenness of the emitted light and the projected light is less than that of type 1, so that LED of type2 has solidified the front runner position of light source for illumination and backlight use. The LED of type2 has expected to be widely spread and further developments have been advanced.
As an example of the improvement in a direction which the development wants to head, there is reported a white luminance LED which is formed by combining: an ultraviolet luminance LED; europium-activated halo-phosphate phosphor or europium-activated aluminate phosphor; copper, aluminum-activated sine sulfide phosphor or europium, manganese-activated aluminate phosphor as green luminance phosphor; and europium-activated yittrium oxysulfide phosphor as red luminance phosphor (for example, refer to non-patent document 1).
As another example of the improvement, there is also reported a white luminance LED which is formed by combining: an ultraviolet luminance LED; europium-activated halo-phosphate phosphor or europium-activated aluminate phosphor; europium, manganese-activated aluminate phosphor as green luminance phosphor; and europium-activated lanthanum oxysulfide phosphor as red luminance phosphor (for example, refer to patent document 1),
Although these white luminance LEDs have a high color rendering property and a high uniformity of the emitted lights that are inherent properties of the white LED of type 2, the luminance is still insufficient and further improvements are required. According to knowledge of the inventors of this invention, in order to attain both high color rendering property and high luminance in the white luminance LED of the above type 2, the following requirements must be fulfilled in a spectrum of the white light. Namely, the emitted lights having a wavelength of about 450 nm, 560 nm, and 620 nm at which respective peaks of color sensibilities for human person exist are contained in a well-balanced manner in the spectrum of the white light, and luminous efficiencies of the respective phosphors of blue (B), green (G) and red (R) luminance components are well balanced.
On the other hand, a liquid crystal display, which uses the above white LED (white luminance diode) as a backlight, has been widely used as a constituting part of mobile communication devices such as cellular phone, mobile phone or the like, OA (office automation) devices such as personal computer or the like, household electrical appliances, audiovisual apparatuses, various switches, various display devices such as light source display plate for backlight or the like.
These conventional liquid crystal displays are formed by combining a backlight using cold cathode fluorescent lamp (CCFL) with color filters, and have improved so as to attain a color reproduction range of about 70% or so in terms of international standard ratio (NTSC ratio). However, further improvement of the color reproduction range has been demanded. Further, since the cold cathode fluorescent lamp so be used as the light source contains harmful mercury, there has been seriously pointed out an adverse impact of the use of mercury on the environment.
Furthermore, in recent years, a blue LED (B-LED) and an LED capable of emitting an ultraviolet light have been developed and commercialized, and an LED lighting apparatus (illumination device) has actively developed. There are three types of LED lighting apparatuses: LED lighting apparatus (type 1) formed by combining B-LED (blue luminance diode) with yellow luminance phosphor; LED lighting apparatus (type 3) formed by mixing the respective lights emitted from R-LED (red luminance diode, G-LED (green luminance diode) and B-LED (blue luminance diode); and LED lighting apparatus (type 2) formed by combining UV-LED (ultraviolet ray emitting diode) with RGB phosphors.
LED lighting apparatuses of type 1 and type 3 have drawbacks such that the color reproducing property is poor and a control of the light color is difficult. Therefore, an expectation to realize LED lighting apparatuses of type 2 composed of UV-LED+RGB phosphors has been increased.
However, even if the white LED composed of UV-LED+RGB phosphors is applied to the backlight source, the color reproducing range was not sufficient. For example, when the white LED disclosed in the patent document 1 is applied to a liquid crystal display, a green color light other than blue color light is mixed into spectrum of a light transmitted through a blue color filter, and a blue color light is mixed into spectrum of a light transmitted through a green color filter. As a result, there is posed problem such that the color reproducing property is disadvantageously lowered.
Patent Document 1: Japanese Patent Application (Laid-Open) No. 2000-73052.
Non-Patent Document 1: Technical Journal of Mitsubishi Densen Kogyo (published July 2002, No. 99)